1. Field
One embodiment of the present invention relates to a magnetic recording medium used for a hard disk drive using a magnetic recording technology, a method of manufacturing the magnetic recording medium, and a magnetic recording apparatus comprising the magnetic recording medium.
2. Description of the Related Art
Higher speed and higher recording density have been required for a magnetic recording apparatus (HDD) to read and write information in accordance with improved processing speed of computers in recent years. The main recording system of a currently available HDD is a longitudinal recording system in which the magnetization direction of the medium is oriented to the in-plane direction. However, a perpendicular recording system is suitable for the purpose of higher recording density, since demagnetizing fields in the vicinity of magnetization transitions are small and sharply reversed magnetizations can be provided in this system. In addition, since the recording layer can be designed to be thicker in the perpendicular magnetic recording medium as compared with the longitudinal magnetic recording medium, degradation due to thermal fluctuation that has been a problem in recent years can be suppressed in this system.
As a perpendicular magnetic recording layer, a CoCr-based alloy magnetic film with an irregular hexagonal crystal structure such as CoCrPt alloy has been mainly studied so far. However, a material with a larger magnetic anisotropy than that of a conventional CoCr-based alloy is desired taking that the thermal fluctuation may be a problem in the perpendicular magnetic recording medium into consideration.
Examples of such a material are ordered alloy materials in which a magnetic element such as Fe or Co and a noble metal element such as Pt or Pd form an ordered phase. It is known, for example, that the ordered alloys of FePt and CoPt with a L10 crystal structure have large magnetic anisotropy of 7×107 erg/cc and 4×107 erg/cc, respectively, in the c-axis direction (i.e., <001> direction) of the crystal lattice. It is expected that perpendicular magnetic recording media having high thermal fluctuation resistance can be provided by using these materials for a recording layer.
However, since these materials have also higher anisotropy field, saturation field and coercivity as well as higher thermal fluctuation resistance, they require an increased recording field for magnetization reversal in writing. Accordingly, even if a currently available writing head is used, sufficient writing is impossible due to insufficient recording field.
In order to solve the problem, tilted perpendicular recording has been proposed in recent years (see, for example, IEEE Transactions on Magnetics, vol. 38, pp. 3675-3683 (2002)). Although magnetic crystal grains in the conventional perpendicular magnetic recording medium are oriented such that the easy axis of magnetization directs to the normal line to the film plane, the magnetic crystal grains in the newly proposed magnetic medium are oriented such that the easy axis of magnetization is tilted from the normal line to the film plane. Since such a magnetic recording medium enables recording on a magnetic recording layer comprising magnetic crystal grains having a larger magnetic anisotropy than that of a conventional magnetic layer with a currently available magnetic head, making it possible to significantly improve thermal fluctuation resistance. Accordingly, if such a magnetic recording medium is fabricated using the above-mentioned ordered alloy material having a large magnetic anisotropy, the magnetic recording medium will be excellent in thermal fluctuation resistance as well as in the signal-to-noise ratio (SNR) of read/write (R/W) characteristics and overwrite (OW) characteristics compared with the conventional ones.
When the above-mentioned ordered alloy material is used for the magnetic recording layer of the tilted perpendicular recording medium, crystal grains thereof should be oriented such that the c-axis as the easy axis of magnetization is tilted from the normal line to the film plane. For example, it is conceivable to align the (111) of (110) plane perpendicular to the film plane. Since the c-axis is perpendicular to the (001) plane, the c-axis is expected to be tilted at an angle of about 56° or 45° relative to the normal line to the film plane when a (111) or (110) orientation film is formed. As has been reported in the above document, the recording field may be reduced to a lowest level when the tilt angle is 45°. An example of (110) orientation film epitaxially grown on a single crystal substrate such as MgO (110) has been known to date. However, no other method for forming the (110) orientation film has been reported. Thus, the current method of manufacturing the (110) orientation film that involves the use of a single crystal substrate is not suitable for the HDD medium in view of the cost. On the other hand, a (111) orientation film can easily be manufactured on a glass substrate. However, since the tilt angle of c-axis in the (111) orientation film is rather large as described above, it is said that the film has a similar structure to the longitudinal magnetic recording medium. Consequently, the demagnetizing fields in the vicinity of magnetization transitions in the medium become larger than those of the perpendicular magnetic recording medium, leading to no improvement in SNR.